Three types of vehicular crash sensors have been considered for use in the deployment of passenger restraint systems such as air bags. One is an electronic sensor which is objectionable because of the high incidence of inadvertent operation arising out of spurious electrical signals associated with an automobile. The second is a spring mass sensor which to date has achieved the widest acceptance. The third is a sensor based on inertial flow of a liquid such as that described in co-pending application Ser. No. 366,427, filed June 4, 1973, now U.S. Pat. No. 3,889,130.
The spring mass sensors currently in use have been effective with crash pulses of extremely short durations. Such pulses are characteristic of head-on crashes or standard barrier impacts. In the case of pulses which are of much longer duration, such as are typical of angular impacts or crashes into energy absorbing guard rails, fences, or wooded areas containing small diameter trees, the spring mass sensors currently utilized may not be capable of functioning with the result that the occupants of the vehicle could be seriously injured. One typical spring mass sensor, for example, underwent a crash involving a 60 m.p.h. velocity change during a time period of 0.2 second, but was unable to effect deployment of the air bag. The occupants of a car equipped with such a sensor would be seriously injured or killed.
Inertial flow crash sensors will function reasonably well for long duration pulses, but they will not provide adequate protection when the velocity change of the vehicle is of extremely short duration. Such sensors, therefore, cannot be utilized on the bumper of a vehicle where the crash pulse tends to be of very short duration. However, they do function well when placed farther back in the vehicle.
Although there is disagreement as to the exact manner in which an optimum crash sensor should function, a sensor which is responsive to a given velocity change of the vehicle, providing the velocity change takes place substantially above a fixed deceleration level, appears to be most satisfactory. Such a sensor should respond to pulses of both short and long duration and should be relatively independent of the shape of the acceleration pulse. One technique by which these characteristics can be achieved is described herein.
In cases where a crash sensor is mounted near the front of the vehicle and receives the maximum pulse corresponding to a given velocity change, spring mass sensors may become excessively large since the travel of the sensing mass must be quite long. In a gas damped crash sensor according to the invention, however, the travel of the sensing mass can be controlled by varying the damping force, thereby resulting in a much smaller and thus less expensive crash sensor.